Virotherapy uses human viruses to kill infected cancer cells by targeting the virus infection and replication to different cancer types. Initially, viral therapy for cancer treatment centered on infecting a small number of target cells with a replicating virus that would replicate, amplify, and spread to adjacent cells, destroying the tumor by a lytic mechanism called oncolysis. For that purpose, adenoviruses have a number of advantages as an oncolytic virus (OV): they achieve a high rate of human cell transduction, can be grown and administered at high titers, and are clinically safe (Koski et al., (2015) Mol. Ther.; Pol et al., (2014) Oncoimmunology 3: e28694; Zheng et al., (2000) Nat. Biotechnol. 18: 176-180).
Oncolytic adenoviruses based on a 24-base pair deletion (D24) in the nucleotide sequence encoding the Rb-binding domain of the immediate early gene E1A, result in a virus deficient for viral replication in normal cells (Fueyo et al., (2000) Oncogene 19: 2-12; Koski et al., Mol. Ther. 18: 1874-1884; Krasnykh et al., (1996) J. Virol. 70: 6839-6846; Suzuki et al., (2001) Clin. Cancer Res. 7: 120-126). Importantly, the 24-base pair deletion does not compromise Ad replication efficiency in cancer cells or oncolytic potency (Fueyo et al., (2000) Oncogene 19: 2-12).
However, these first-generation conditionally replicating adenoviruses (CRAds) had limited efficacy as the therapeutics were based on the Ad5 serotype. The primary receptor for Ad5 is the Coxsackievirus B and Adenovirus Receptor (CAR) that is poorly expressed on most cancer cells (Miller et al., (1998) Cancer Res. 58: 5738-5748). Furthermore, CAR is highly expressed on normal lung and liver cells severely limiting the potential usefulness of the virus (Bauerschmitz et al., (2002) Int. J. Oncol. 21: 1161-1174; Bergelson et al., (1997) 275: 1320-1323). Although these CRAds lacked efficacy in failing to infect an adequate number of cancer cells, the therapeutics were proven safe for patient treatment (Russell et al., (2012) Nat. Biotechnol. 30: 658-670).
A second approach has been to construct a recombinant chimera fiber with the knob domain replaced with that from another Ad serotype. As an important region responsible for binding to cell surface receptors, the fiber knob domain represents a major determinant of Ad tropism. For example, replacing the Ad5 knob with the Ad3 knob, resulting in Ad5/3 serotype chimera, has proven successful in re-targeting the vector to cells with low levels of CAR, but high levels of the Ad3 receptor(s) that are upregulated on a number of cancer cell types (Koski et al., Mol. Ther. 18: 1874-1884; Krasnykh et al., (1996) J. Virol. 70: 6839-6846; Kanerva et al., (2002) Clin. Cancer Res. 8: 275-280; Kimball et al., Clin. Cancer Res. 16: 5277-5287).
This infectivity enhancement has become an area of research focus as Ad3 binding to the Desmoglein 2 (DSG2) receptor, a primary receptor for the Ad3 serotype, improves the tumor microenvironment for drug delivery as the interaction results in a signaling cascade that releases the tight epithelial adhesion that normally precludes white blood cell and therapeutic agent tumor penetration (Liu et al., (2014) Gynecol. Oncol. 132: 722-729; Wang et al., Nat. Med. 17: 96-104). The Ad5/3 modification serves a second important function by avoiding the coxsackievirus and adenovirus receptor that exists on many normal cells allowing improved tumor selection. Hence, the use of this virus has numerous potential therapeutic benefits. As with the Ad5-RGD D24 virus, the Ad5/3 D24 vector also exhibits a substantial improvement in the transduction efficiency of certain cancer cells (Pesonen et al., (2011) Mol. Pharm. 8: 12-280).